Materials that can be stimulated to emit light, commonly referred to as luminescent materials or luminophors, can be employed in a wide variety of applications. In a simple application a luminophor can be employed to absorb ultraviolet radiation and emit visible light. In other applications one of a variety of different stimulating sources of radiation, such as electrons, .alpha. or .beta. particles, .gamma. rays or X rays, can be employed. In still another type of application, referred to as electroluminescence, the recombination of hole-electron pairs can provide the stimulating energy source for luminescence.
For many luminophor applications there is a need for thin (&lt;1 .mu.m) film luminophors. Specific examples of luminophors used to construct thin films are provided by VanSlyke et al U.S. Pat. No. 4,539,507, Tang et al U.S. Pat. No. 4,769,292, VanSlyke et al U.S. Pat. No. 4,720,432, Tang et al U.S. Pat. No. 4,885,211, Perry et al U.S. Pat. No. 4,950,950, VanSlyke U.S. Pat. No. 5,150,006 (22 Sep. 92), Van Slyke U.S. Pat. No. 5,151,629, and Bryan et al U.S. Pat. No. 5,141,671 (25 Aug. 92).
In U.S. Pat. No. 5,150,006, Van Slyke et al. U.S. Pat. Nos. 5,151,629, and 5,141,671, two 2-methyl-8-hydroxyquinoline (2MQ) ligands were complexed to aluminum. In U.S. Pat. No. 5,151,629, highly efficient electroluminescent devices were described comprising an oxygen bridged complex with structure (2MQ).sub.2 AlOAl(2MQ).sub.2 as the emitting component. U.S. Pat. No. 5,141,671, teaches a structure (2MQ).sub.2 AlOAr where Ar represents a phenyl or substituted phenyl moiety that has a lower efficiency but higher stability. The tris complex Al(2MQ).sub.3 could not be formed by the method described in U.S. Pat. No. 5,141,671, because three 2MQ ligands could not coordinate effectively to a single aluminum.
In P. R. Scherer, Q. Fernando, "Reaction of 2-Methyl-8-Quinolinol with Aluminum (III) in Nonaqueous Media," Anal. Chem. 40(13), 1938(1968), the authors describe the preparation of a 1:1 adduct of Al(2MQ).sub.3 and DMSO. The tris complex Al(2MQ).sub.3 was not isolated, and the structure of the adduct was not determined conclusively. Fluorescence by the adduct sufficient for electroluminescent applications was not reported.
Until the discovery of the (2MQ).sub.2 AlOAr compounds, no isolatable fluorescent metal chelates of Al with 2-methyl-8-hydroxyquinoline other than (2MQ).sub.2 AlOAl(2MQ).sub.2 were known. Unfortunately, the synthetic methodology used to prepare (2MQ).sub.2 AlOAr is limited, precluding the synthesis of aluminum chelates of 2-methyl-8-hydroxyquinoline with a third ligand other than a phenol which was contemplated to be useful as an emitting compound in electroluminescent devices. Attempts to prepare, for example, a compound by reacting benzoic acid rather than phenol with aluminum isopropoxide and 2-methyl-8-hydroxyquinoline resulted, by the methods of preparation described in the above patents, in the formation of the (2MQ).sub.2 AlOAl(2MQ).sub.2 compound.
One application in which the novel composition of this invention can be used is electroluminescent devices. Electroluminescent devices (hereinafter also referred to as EL devices) contain spaced electrodes separated by an electroluminescent medium that emits light in response to the application of an electrical potential difference across the electrodes.
In currently preferred forms organic EL devices are comprised of an anode, an organic hole injecting and transporting zone in contact with the anode, an electron injecting and transporting zone forming a junction with the organic hole injecting and transporting zone, and a cathode in contact with the electron injecting and transporting zone. When an electrical potential is placed across the electrodes, holes and electrons are injected into the organic zones from the anode and cathode, respectively. Light emission results from hole-electron recombination within the device.